CH701 Sans Slats?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Lee Schaumberg

Well-Known Member
Joined
Aug 28, 2003
Messages
215
Location
Northern Wisconsin
Hello
Unable to edit post #19 I will explain WHY removing the slats makes the tail lighter. As the wing chord is shortened (removal of slats) the cg moves forward and lightens the tail. Rather than removing the slats to make the tail lighter just load the plane with a more forward cg! What is needed is an accurate comparison of cgs to see how cg changes the performance.
 
Last edited:

Tom Nalevanko

Well-Known Member
Joined
Oct 10, 2007
Messages
1,316
Location
Alpine, WY
If the slats are ahead of the CG, (and indeed they are) then removing them moves the intitial CG aft (rearward). So your explanation does not seem correct.

I have no idea what "lighter tail" means.... Nothing is being removed from the tail so it is neither lighter or heavier. We need to have some commonly accepted aircraft design terms in use here to have some real understanding....
 

orion

Well-Known Member
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
I think this is partly an issue of terminology. When you remove the fixed slats, the basis for the MAC changes from the full chord with the slats to one where you just have the basic wing planform with a shorted chord. If the physical CG of the airplane remains at essentially the same physical position (the slats weight hardly anything), the airplane is now just a hair nose heavier since the CG position on the new MAC is now positioned a bit further forward, as represented by a percentage.

However, contrary to the previous post, this actually puts more load on the tail, not less. In order to achieve a stable airplane the tail usually needs to develop a downward load to counter the wing's pitching moment, which is oriented in a nose-down direction. The downward load also needs to counter the moment that is generated by the couple that is formed between the resultant lift vector and the CG position. Without the slats the resultant lift vector moves aft, thus generating a slightly larger moment couple with the existing CG position.

And there is one other incremental increase on the tail, one which has to do with the new shape of the wing's section. In general, leading edge devices tend to reduce the wing's pitching moment - by removing the slats the airplane wing's moment coefficient is now increased so the tail has to work harder in order to generate the countering force.

In short, no part of the mod unloads the tail - all the factors actually increase the load at the tail.
 

rogwibb

New Member
Joined
Aug 20, 2009
Messages
1
Location
Atwater, Ca. /US
I am currently building a 701 but haven't made a decision concerning slats vs VG's or the engine. I built an X-Air with a Jabiru 2200 and liked the engine, but was wondering what your opinion was of it was on the 701, as well as your opinion regarding building without slats.

Thank for any help

Roger
Atwater, Ca.
 

Kmccune

Well-Known Member
Joined
Aug 5, 2007
Messages
162
Hi Roger,
I would suggest that you look up this topic on the Matronics Zenith and CH701/CH801 forum. Then contact the guys who have done it. I have spoken to several of them and they all say they would never put the slats back on. In addition the guy selling them, John Gilpin does have pretty good info on his website. He is not making tons of money and does not expect to, from this small market. CH did say he has no problems with removing the slats from an engineering standpoint. And his predictions of the loss of STOL capability are not correct, at least not to the point that he stated.

My 701 wing is almost ready to assemble and there are no slats or slat bracket in the pile. Oh and my spars are longer too CH did approve this.

Kevin
 

mcjon77

Well-Known Member
Joined
Aug 5, 2008
Messages
205
....Oh and my spars are longer too CH did approve this.

Kevin
Kevin,

Just wondering why you made your spars longer. I am assuming you are making a longer wing. What types of benefits have been reported? I vaguely remember something about pegastol wings, but don't remember why they were used.

Thanks.
 

dirtstrip

Well-Known Member
Joined
Dec 18, 2008
Messages
64
Location
Conde, SD. on the farm
I checked into Pegastol wings when I had my 701. The slats moved in and out like the ones on a Helio when the angle of attack got high enough they caught air and came out but at cruise angle of attack they moved back toward the wing and the slat closed. A very expensive modification to gain cruise by eliminating drag of the slats. I would have had to buy two new wings and I was quoted $15000!
One point on the Jabiru engine. It is a good engine from everyone that has one but it is a direct drive high rpm engine. I have always read that most stol planes need lots of torque on the prop and their bigger props perform better at lower rpms. The higher speed short props generally work better with faster cruise applications rather than climb and short field takeoff. When you check with owners be sure you are asking about performance in your type aircraft.
 

DLrocket89

Well-Known Member
Joined
Aug 10, 2009
Messages
249
Location
Janesville, Wi
Just out of curiosity, has anyone done the remove slat/install VG thing on the CH801? The 801 is pretty much what I'm looking for in an airplane, but I was hoping for a bit higher cruise speed. The other airplane I'm interested in is the Bushcaddy L164, if that helps.

I'm interested in flying amphibious floats from an airport to lakes for camping/fishing expeditions.
 

Kmccune

Well-Known Member
Joined
Aug 5, 2007
Messages
162
Sorry,
Yes the wing span will be longer, With the VGs and longer wing you get , better glide, no discernible take off difference, including climb out angle and 20% less fuel burn (this is why its so good at STOL landings, its a rock with wings) you can drag it in with power or nice and easy flat and slow and the stall is that same mush, with no wing drops or nasty stuff. But what Chris states on the slats vs VGs site posted above is true, you will not loose altitude as quickly with out slats ( approach over obstacles), even more so with longer wings. But the slips is available and with the701s all flying rudder and big flat sides the 701 is very good at this.
But really do you need to drop like a rock when you cut power...do you want too?

The long wing will also give me better wing loading as mine will be a little on the heavy side.

For some guy doing jungle opps some place the 701, as delivered is very good, a long wing 701 with slats would be better. But I have no intention of flying in or out of 200' clearings with trees on ether end. I'll take the 20% less fuel burn or faster cruise and love it.

Please note that I think the 701 is cool as designed , its just not quite what I want, so I'm changing it just a touch. Your wants may vary.

The link below is a Corvair powered airplane on its maiden flight, the pilot is a high time experienced test pilot. He cut the power over the end of the runway. Of course you can keep just a trickle of power and land normally all the time, this is just to show how STOL the 701 can be... you decide.

[video=youtube;lXaLC8h0WIk]http://www.youtube.com/watch?v=lXaLC8h0WIk[/video]


Kevin




Kevin,

Just wondering why you made your spars longer. I am assuming you are making a longer wing. What types of benefits have been reported? I vaguely remember something about pegastol wings, but don't remember why they were used.

Thanks.
 
Last edited by a moderator:

Procreator

Member
Joined
Oct 16, 2009
Messages
6
Location
Madrid, Spain
A few months back while lurking on the forum I ran across this thread, this was the first I had heard about the idea of substituting slats for VG's. Now that I can speak from personal experience I'll share my findings.

First I do not sell slats or VG's and I have no affiliation with any seller of either so I stand nothing to gain or lose by whatever decision you take.

Once turned on to the prospect of removing the slats of my Savannah I looked around for additional information on the use of VG's and which product to buy. I opted for the Stolspeed VG's because of the information available and for the simplicity of the design and their low cost.

I removed the slats, installed the VG's and flew the plane. Even with all of the positive reports the first time I took to the air I had my doubts how it was going to handle. Onlookers had even more doubts and suggested a crow hop, once off the deck on a low pass it was clear that the handling was fine.

After having flown the plane for about 80 hours in all types of conditions I can can confidently recommend to any one to discard the slats. Forget the scare tactics published by those who manufacture slats, or claim that the standard airfoil must have the leading edge modified to be safe... it's all crap.

Removing the slats does not transform a STOL machine into a high speed cruise machine, you get a gain of about 10kmh with improved fuel economy and makes the aircraft easier to land. Stall speed may increase by 1 or 2kmh in still air tests but in practical use I can't tell any differance. In cruise the nose rides somewhat lower than with the slats and the flaps are more effective. Slats do their thing at high angles of attack and flaps lower the nose so they are antagonistic. Climb is also improved by some 200fpm.

Rather than me writing a thesis all I can say is after comparing my own expirence with the test results published on the Stolspeed site I find those results to be trustworthy and a good reference.

I'm glad I went this route and I would not consider replacing the slats. In fact ICP, the manufacturers of the Savannah and Bingo no longer offer an aircraft with fixed slats, these models now use VG's.

So if you're concerned that by removing your slats you're going to screw up your plane, don't worry you're not.

Fly safe,
Steve
 

Jman

Site Developer
Joined
Oct 22, 2002
Messages
1,881
Location
Pacific NW, USA!
Thanks for the report Steve. In your research did you happen to see if an 801s had successfully swapped VGs for Slats?
 

mcjon77

Well-Known Member
Joined
Aug 5, 2008
Messages
205
Any idea how much of an improvement in glide ratio removing slats provides? I seem to recall that with slats the reported glide ratio was something like 6 or 7 to 1.
 

bmcj

Well-Known Member
HBA Supporter
Joined
Apr 10, 2007
Messages
13,252
Location
Fresno, California
Any idea how much of an improvement in glide ratio removing slats provides? I seem to recall that with slats the reported glide ratio was something like 6 or 7 to 1.
I think the elevator in the Empire State Building gets close to that number, but it carries more people.
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
12,821
Location
Port Townsend WA
What's so bad about a 7 to 1 glide for a bush plane? Just about perfect for a short steep glide into a tight landing area.

I think if the slats are removed, maybe a leading edge cuff should be installed. Otherwise known as a "droop snoot". Has anyone done this?
 

Procreator

Member
Joined
Oct 16, 2009
Messages
6
Location
Madrid, Spain
I can't tell you Jake, I only know one guy around here with an 801 and he still has the slats on.

There are however several Savannahs, Bingos, Land Africas and 701 flying without the slats and in speaking with their pilots I have yet to hear anyone report anything negative. One pilot had yet to install the VG's or remove the mounting brackets and he is not going back to slats.

Apparently it's the thick airfoil that gives these planes their STOL characteristics, the aerodynamic supplements only help a little at the extreme angles of attack. Both supplements work, the difference is that VG's offer much less drag, if they offer any perceivable drag at all.

The drag generated by the slats varies with the angle of attack, that's why it's not unusual to see bent main gear on these planes. If you don't hold some power during the flare as the nose comes up the speed deteriorates so fast that you either set it down immediately like you're on an aircraft carrier or you suddenly just drop if you try to hold it off the deck as you would a non STOL plane.

Even without the slats I leave a little power on, especially if I've got a passenger. Slats are not the only high drag element on these aircraft, there's the thick wing, boxy fuselage, double wing struts and the big tires. Only these generate more drag as speed goes up rather than when speed goes down and the exponential increase in drag is not as radical as the slats.

One of the arguments in favor of slats is that their increased drag lets you make a steeper approach without the speed build up, if slats are removed you will not be able to enter into as tight of an area. I wouldn't consider this an issue in the least. Approaching at idle will have you dropping like a rock, the prop acts as an airbrake. Since flaps are more predictable and funtional without the slats you can use those, too. Full flap feels like you've deployed a drag chute. And then of course you can always side slip.

Maybe an alternative for the die-hard slat enthusiast out there would be a slat like the one being used on the Mackey SQ2 (see YouTube). It moves automatically in conjunction with the air presure opening and closing so the drag is reduced when not needed. Since it doesn't actually retract I would imagine that there are CG issues to consider because when the slat is closed it now forms part of the airfoil, by moving the leading edge forward you have in essence moved the CG back.

But then again my buddies that think I'm crazy for having removed the slats, me not being an aeronautical engineer, would not be willing to try this unless it is developed by a qualified source.

I'm satisfied with the plane as it is so I'm set.

Regards,
Steve
 

bmcj

Well-Known Member
HBA Supporter
Joined
Apr 10, 2007
Messages
13,252
Location
Fresno, California
What's so bad about a 7 to 1 glide for a bush plane? Just about perfect for a short steep glide into a tight landing area.
Nothing at all. Just fodder for jokes (especially from sailplane pilots). A steep glide helps get over obstacles and into short fields, but it is not a requirement for STOL nor does it make a STOL plane. Many planes with good L/D are short field capable and many planes with steep glides (aka, Space Shuttle) are not STOL aircraft (OK, so it might be STOL in the take-off phase :gig:).
 
2
Group Builder
Top